The polymerization of hemoglobin S molecules into fibers and alignment of these fibers to form a crystalline phase (gelation) rigidifies the red cell and produces the distortion referred to as sickling. In addition to its importance in disease this polymerization-crystallization phenomenon provides an excellent example of a macromolecular assembly process. The aim of this project is to gain a basic physical-chemical understanding of the sickling phenomenon and to interpret the behavior of potentially therapeutic additives in terms of their effects upon the kinetics and thermodynamics of the gelation of hemoglobin S.